1. Field of Invention
The present invention relates to micro-electro-mechanical system (MEMS) microphone chip and more particularly to a MEMS microphone chip with an expanded back chamber.
2. Related Art
In the wake of rapid development of semi-conductor technology, electronic products are becoming slimmer and more compact in design than ever before. The integration of microphones in semi-conductor industry to convert sound waves into electronic signals is the faster developing technology in the electroacoustic field. Many electronic products found in the market today are installed with MEMS microphones, which are more heat-resistant, anti-vibrational, and radio frequency interference (RFI) resistant than conventional electret condenser microphones (ECM) which are more widely used. Because of its better heat-resistant characteristic, the MEMS microphone can be manufactured by automatic surface mount technology (SMT), therefore production procedures are simplified, production costs are reduced, free designs are allowed and system costs are reduced.
Referring FIG. 1, it shows a cross-sectional view of a conventional MEMS microphone chip. The conventional MEMS microphone chip is formed in this way: A silicon oxide insulating layer 11 and a silicon nitride insulating layer 12 are formed on a silicon base plate 10 by microelectromechanical manufacturing process; a vibration membrane layer 13 and an electrode 14 are formed on the silicon nitride insulating layer 12, and a conducting wire 15 is connected between the vibration membrane layer 13 and the electrode 14; furthermore, a chamber 16 is formed in the silicon base plate 10 by etching, so that the vibration membrane layer 13 is suspended on the silicon nitride insulating layer 12; the conventional MEMS microphone chip can be disposed on a bottom plate, and connected electrically to a semi-conductor chip (ASIC-Application Specific Integrated Circuit) on the same bottom plate; then a MEMS microphone is formed and assembled after the bottom plate is fitted with an outer case with sound holes. The vibration membrane layer 13 vibrates in response to external sound waves which are transmitted to the MEMS microphone chip through the sound holes; then an electronic signal is correspondingly produced and is transmitted to the semi-conductor chip via the electrode 14, it is then output to a processor of an electronic product installed with the MEMS microphone.
The space of the chamber 16 formed in the silicon base plate 10 is very small because of a micro-size of the MEMS microphone chip, thus the vibration force of the vibration membrane layer 13 is reduced due to the air resistance produced by the limited space of the chamber 16. This causes the deterioration of sound quality of the MEMS microphone, especially in terms of sensitivity. Furthermore, in a process of putting adhesive on the abovementioned conventional MEMS microphone chip to be coupled to the bottom plate, the opening of the chamber 16 has to be avoided, therefore it is rather troublesome in manufacturing and the time cost will be increased.